The present invention relates to the field of riser termination devices and more particularly to the field of separating mixed phase gas-solid streams in hydrocarbon cracking units. The present invention also relates to improved stripping operations in hydrocarbon cracking units, such as fluid catalytic cracking (FCC) units.
Chemical reaction systems utilizing solids in contact with gaseous or vaporized feedstocks have long been employed. The solids may participate in the reaction as a catalyst, provide heat required for an endothermic reaction, or both. Alternatively, the solids may provide a heat sink required for an exothermic reaction. The terms, solid and catalyst are used interchangeably herein. Similarly, the terms gas and vapors are used interchangeably herein.
In the past, cracking of petroleum products was performed in fluidized bed reactors that had as an advantage a relatively isothermic temperature profile. However, as catalysts improved and residence times decreased, the bed depth became shallower and increasingly unstable. For this reason tubular reactors employing solid-gas contact in pneumatic flow were developed and have been used with great success particularly in the catalytic cracking of hydrocarbons to produce gasoline products where reactor residence time is less than 0.5 to 5 seconds, optimally less than 2 seconds.
In general, catalytic cracking of relatively high boiling hydrocarbons to form substantial quantities of material boiling in the gasoline range is carried out in the following sequence as described in Pfeiffer et al., U.S. Pat. No. 4,756,886, which is incorporated herein by reference: hot regenerated catalyst is contacted with a hydrocarbon feed in a reaction zone under conditions suitable for cracking; the cracked hydrocarbon gases are separated from the spent catalyst using conventional cyclones and the spent catalyst is steam stripped to remove hydrocarbons and subsequently fed to a regeneration chamber where a controlled volume of air is introduced to burn the carbonaceous deposits from the catalyst, and the regenerated catalyst is returned to the reaction zone.
A problem with these fluidized catalytic cracking systems has been obtaining rapid and efficient separation of the gas and solid phases. Another problem has been the efficient re-aeration and stripping of separated spent catalyst particles.
Previous attempts have been made in the art to separate the phases by use of centrifugal force or deflection means. Nicholson, U.S. Pat. No. 2,737,479, combines reaction and separation steps within a helically wound conduit containing a plurality of complete turns and having product draw-offs on the inside surface of the conduit to separate solids from the gas phase by centrifugal force. Solids accumulate on the outside of the conduit, while gases concentrate at the inner wall, and are removed at the draw-offs. The Nicholson unit produces a series of gas product streams each in a different stage of feed conversion due to the multiple product draw offs that cause varying exposure time of the gas to the reaction conditions.
Ross et al., U.S. Pat. No. 2,878,891, attempted to overcome this defect by appending to a standard riser a modification of Nicholson""s separator. Ross et al. teach a separator comprised of a curvilinear conduit making separation through a 180xc2x0 to 240xc2x0 turn. Centrifugal force directs the heavier solids to the outside wall of the conduit allowing gases that accumulate at the inside wall to be withdrawn through a single drawoff. While the problem of various stages of conversion of the product is decreased, other drawbacks of the Nicholson unit are not eliminated.
Both devices effect separation of gas from solids by changing the direction of the gas 90xc2x0 at the withdrawal point, while allowing solids to flow linearly to the separator outlet. Because solids do not undergo a directional change at the point of separation, substantial quantities of gas flow past the withdrawal point to the solids outlet. For this reason, both devices require a conventional separator at the solids outlet to remove excess gas from the solid particles. However, product gas removed in the conventional separator has remained in intimate contact with the solids, and is, therefore, severely degraded.
Another drawback of these devices is the limitation on scale-up to commercial size. As conduit diameter increases, the path traveled by the mixed phase stream increases proportionately so that large diameter units have separator residence times approaching those of conventional cyclones. Increasing velocity can increase residence time, but as velocities exceed 60 to 75 ft/sec, erosion by particles impinging along the entire length of the curvilinear path becomes progressively worse. Reduction of the flow path length by decreasing the radius of curvature of the conduit also reduces residence time, but increases the angle of impact of solids against the wall thereby accelerating erosion.
Pappas, U.S. Pat. No. 3,074,878, devised a low residence time separator using deflection means wherein the solid gas stream flowing in a tubular conduit impinges upon a deflector plate causing the solids, which have greater inertia, to be projected away from a laterally disposed gas withdrawal conduit located beneath said deflector plate. Because solids do not change direction while the gas phase changes direction relative to the inlet stream by only 90xc2x0 there results an inherently high entrainment of solids in the effluent gas. While baffles placed across the withdrawal conduit reduce entrainment, these baffles as well as the deflector plate are subject to very rapid erosion in severe operating conditions of high temperature and high velocity. Thus, many of the benefits of the separators of the prior art are illusory because of the limitations in their efficiency, operable range, and scale up potential.
Gartside et al., U.S. Pat. Nos. 4,288,235, 4,348,364 and 4,433,984, disclosed an apparatus for rapidly separating particulate solids from a mixed phase solids-gas stream from tubular type reactors. Separation is effected by projecting solids by centrifugal force against a bed of solids as the gas phase makes a 180xc2x0 directional change. The solids phase, however, is required to undergo two 90xc2x0 degree changes before exiting the apparatus.
Larson, U.S. Pat. No. 3,835,029, discloses a downflow catalytic cracker entering a cylindrical separator with a series of openings in the outside wall through which the hydrocarbon passes. The catalyst solids pass downwardly to a stripper section and then into a regenerator. Within the equipment and spatial constraints, the separator of Larson is limited because there is no progressively increasing lateral flow path as a function of the height of the openings to help effectuate separation once the mixed phase gas solids stream enters the separator.
Pfeiffer, U.S. Pat. No. 4,756,886, teaches a rough cut separator that has a frusto-conical chamber having substantially conical walls tapering downwardly and outwardly and means defining at least one opening in said conical walls for conveying solids free gas.
Other more recent separators are disclosed in Ross et al., U.S. Pat. No. 5,259,855; and Van der Akker et al., U.S. Pat. No. 4,961,863. Although these separation devices have met with some success, there still exists a need in the art for more improved devices, especially those which can facilitate stripping of hydrocarbons from the catalyst particles prior to regeneration.
Accordingly, such an improved riser termination device is provided by the present invention. The device of the present invention comprises:
a) an integrated three stage catalyst separation unit for separating solids from gas, the separation unit comprising:
(1) a first separation stage comprising: (i) a substantially cylindrical inlet downcomer pipe having an inlet, and a downcomer having a vertical window located in a lower portion of the downcomer; (ii) a catalyst distribution plate operatively connected to said downcomer floor; (iii) a vapor outlet duct located operatively connected to the window and extending upwardly and concentric to the downcomer pipe; and (iv) a catalyst outlet operatively connected to the catalyst distribution plate;
(2) a second catalyst separation stage comprising a short radius curvature at the downcomer window outlet opening into the vapor outlet duct that allows the gases to separate from the catalyst plated out against the vapor outlet duct wall and turn upwards while the plated out catalyst slows down and exits through a shave-off window and drops through an internal dipleg to the catalyst distribution plate along with the catalyst from the first catalyst separation stage; and
(3) a third catalyst separation stage comprising a catalyst vent formed by a converging channel formed between the back of the first stage downcomer and the vapor outlet duct inner wall above the internal catalyst dipleg; and
b) a crossover duct for connecting a riser reactor to the inlet of the downcomer.
A further embodiment of the present invention provides a riser termination device further comprised of a bath tub for receiving separated catalyst from the catalyst outlet for re-aerating the separated catalyst.
The present invention also provides a bath tub device for re-aerating catalyst separated from cracked product vapors in a separator comprising:
a containment means for receiving and containing catalyst from the separator;
an aeration means for fluidizing catalyst contained in the containment means;
a fluidized catalyst outlet for allowing fluidized catalyst to exit the containment means; and
a support means for supporting the containment means in a riser disengager vessel.
In preferred embodiments, the aeration means injects steam into the catalyst thereby allowing dilute phase stripping of the catalyst to occur as the fluidized catalyst exits the containment means, and thereby providing an improved spent catalyst stripping system and process.
These and other objects of the invention will be apparent to those of ordinary skill in the art from an inspection of the specification, figures and claims.